Transmit power allocation for adaptive multi-carrier multiplexing mimo systems

ABSTRACT

The present invention relates to transmit power allocation in multi-carrier, multiplexing MIMO communication systems. The present invention especially relates to a MIMO communication device, a method of assigning transmit power to two or more communication channels and a software program product. A multiple-input-multiple-output, MIMO, communication device according to the present invention comprises a link controller adapted to assign transmit power to two or more transmission channels, each of said transmission channels having preassigned a portion of transmit power for each of a group of subcarriers, said link controller being further adapted to assign, for each subcarrier of said group of subcarriers, at least part of the preassigned transmit power portion of a transmission channel that is not used for transmitting information at the subcarrier, to one or more transmission channels that are used for transmitting information at the subcarrier.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.12/275,728, filed Nov. 21, 2008, the entire contents of which isincorporated herein by reference, and claims the priority under 35U.S.C. §119 to European Application No. 07 123 861.2, filed Dec. 20,2007.

FIELD OF THE PRESENT INVENTION

The present invention relates to transmit power allocation inmulti-carrier, multiplexing MIMO communication systems. The presentinvention especially relates to a MIMO communication device, a method ofassigning transmit power to two or more communication channels and asoftware program product.

DESCRIPTION OF THE STATE OF THE ART

Multiple-input-multiple-output (MIMO) communication systems use aplurality of transmit ports (e.g. transmit antennas) and receive ports(e.g. receive antennas). Multiplexing MIMO systems, which are also knownas spatial multiplexing MIMO systems, split an incoming data stream onseveral transmission channels, resulting in an increased data rate.(Alternatively, a higher robustness may be obtained instead of a higherdata rate.) In simple systems, the transmission channels may correspondto the transmit ports, in more complex systems an encoding (precoding)is applied by a multiplexing MIMO precoder in the transmitter which,typically, spreads the transmission channels over the transmissionports. A decoding (detection) corresponding to the encoding is appliedby a multiplexing MIMO detector in the receiver in order to recover thetransmission channels. An example of such encoding is Eigenbeamforming.Multi-carrier modulation schemes (e.g. OFDM or multi-carrier waveletmodulation) are using a plurality of subcarriers in order to transmitdata. In adaptive multi-carrier communication systems the modulationscheme for each subcarrier is chosen based on a signal-to-noise ratio(SNR) of the subcarrier. Waterfilling is a known method for transmitpower allocation which optimizes the overall throughput (data rate)while holding the overall transmit power below a maximum value.Waterfilling is efficient for non-adaptive multi-carrier systems (i.e.when all subcarriers are modulated in the same way). For adaptivemulti-carrier systems, however, waterfilling shows almost notransmission throughput gain.

The problem to be solved by the present invention is to provide for aMIMO communication device and a method and a computer program productfor assigning transmit power allowing for a higher data rate.

BRIEF DESCRIPTION OF THE PRESENT INVENTION

This problem is solved by a MIMO communication device according to claim1 of the present invention, the method of assigning transmit power totwo or more communication channels according to claim 10 of the presentinvention and the software program product according to claim 17 of thepresent invention.

The MIMO communication device according to the present inventioncomprises a link controller adapted to assign transmit power to two ormore transmission channels, each of said transmission channels havingpreassigned a portion of transmit power for each of a group ofsubcarriers, said link controller being further adapted to assign, foreach subcarrier of said group of subcarriers, at least part of thepreassigned transmit power portion of a transmission channel that is notused for transmitting information at the subcarrier, to one or moretransmission channels that are used for transmitting information at thesubcarrier.

The MIMO communication device advantageously comprises a multiplexingMIMO detector, whereby said transmission channels correspond to outputports of the multiplexing MIMO detector.

Advantageously, said link controller is adapted to determine anindicator value of a signal-to-noise ratio, SNR, for each subcarrier ofsaid group of subcarriers on each transmission channel and to determine,for each transmission channel and each subcarrier of said group ofsubcarriers, if the transmission channel is used for transmitting dataat the subcarrier based on the corresponding indicator value of the SNR.

Advantageously, said link controller is adapted to determine for atleast one subcarrier on at least one transmission channel an indicatorvalue of an expected SNR, said indicator value of the expected SNR beingbased on the corresponding indicator value of the SNR and thecorresponding assigned transmit power.

Advantageously, the MIMO communication device comprises a symboldemapper unit for demodulating received symbols, whereby the linkcontroller is adapted to determine a demodulation scheme for each ofsaid at least one subcarrier on said at least one transmission channelbased on the corresponding indicator value of the expected SNR and toconfigure said symbol demapper unit with the determined demodulationschemes so that a symbol transmitted on a given one of said at least onesubcarrier on said at least one transmission channel is demodulated withthe corresponding determined demodulation scheme.

Alternatively to comprising a multiplexing MIMO detector, the MIMOcommunication device advantageously comprises a multiplexing MIMOprecoder, whereby said transmission channels correspond to input portsof the multiplexing MIMO precoder.

Advantageously in this case, the MIMO communication device comprises areceiving unit adapted to receive notching information indicating whichtransmission channels are not used for transmitting information at whichsubcarriers, whereby the link controller is adapted to determine whichtransmission channels are not used to transmit information at whichsubcarriers based on the received notching information.

Advantageously, at least some of said transmission channels havepreassigned the same transmit power portions for a given subcarrier ofsaid group of subcarriers.

Advantageously, at least some subcarriers of said group of subcarriershave preassigned the same transmit power portions for a giventransmission channel.

The method of assigning transmit power to two or more communicationchannels according to the present invention is a method wherein each ofsaid communication channels has preassigned a portion of transmit powerfor each of a group of subcarriers and comprises a step of assigning,for each subcarrier of said group of subcarriers, at least part of thepreassigned transmit power portion of a transmission channel that is notused for transmitting information at the subcarrier, to one or moretransmission channels that are used for transmitting information at thesubcarrier.

Advantageously, said communication channels correspond to input ports ofa multiplexing MIMO precoder and/or output ports of a multiplexing MIMOdetector.

Advantageously, the method further comprises steps of determining anindicator value of a signal-to-noise ratio, SNR, for each subcarrier ofsaid group of subcarriers on each transmission channel, and determining,for each transmission channel and each subcarrier of said group ofsubcarriers, if the transmission channel is used for transmitting dataat the subcarrier based on the corresponding indicator value of the SNR.

Advantageously, the method further comprises a step of determining forat least one subcarrier on at least one transmission channel anindicator value of an expected SNR, said indicator value of the expectedSNR being based on the corresponding indicator value of the SNR and thecorresponding assigned transmit power.

Advantageously, the method further comprises steps of determining amodulation scheme for each of said at least one subcarrier on said atleast one transmission channel based on the corresponding indicatorvalue of the expected SNR and modulating each given one of said at leastone subcarrier on said at least one transmission channel with thecorresponding determined modulation scheme.

Advantageously, at least some of said transmission channels havepreassigned the same transmit power portions for a given subcarrier ofsaid group of subcarriers.

Advantageously, at least some subcarriers of said group of subcarriershave preassigned the same transmit power portions for a giventransmission channel.

The software program product according to the present is adapted tocarry out the method according to the present invention when executed byone or more processing devices.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram showing two communication devices accordingto an embodiment of the MIMO communication device according to thepresent invention.

FIG. 2 shows a flow diagram corresponding to an embodiment of the methodof assigning transmit power to two or more transmission channelsaccording to the present invention.

FIG. 3 shows examples of a SNR increase obtained by the embodiment ofthe method of assigning transmit power according to the presentinvention.

FIG. 4 shows a first realization of the embodiment of the method ofassigning transmit power according to the present invention whenexecuted in a receiver.

FIG. 5 shows a second realization of the embodiment of the method ofassigning transmit power according to the present invention whenexecuted in a transmitter.

FIG. 6 shows a flow diagram of a further embodiment of the method ofassigning transmit power to two or more transmission channels accordingto the present invention.

DESCRIPTION OF DETAILED EMBODIMENTS

The general idea of the present invention, which is to allocate transmitpower that is preassigned to a subcarrier of a first transmissionchannel that is not used for transmitting data to one or moretransmission channels that are used for transmitting data on thissubcarrier, is now explained with reference to specific embodiments ofthe present invention.

The transmission channels of the present invention may directlycorrespond to transmit ports of a MIMO transmitter or may correspond totransmission channels (e.g. decoupled transmission channels obtained byEigenbeamforming) obtained by a means of multiplexing MIMO precoding atthe transmitter and corresponding decoding at the receiver.

FIG. 1 shows a communication system 6 comprising two communicationdevices 1-1, 1-2 according to an embodiment of the present invention.The communication device 1-1 comprises a transmitting unit 2-1, areceiving unit 3-1 and a link controller 4-1. The communication device1-2 comprises a transmitting unit 2-2, a receiving unit 3-2, and a linkcontroller 4-2. The communication devices 1-1 and 1-2 are identical. Thecommunication devices 1-1, 1-2 may both operate as receiver and astransmitter. In the situation depicted in FIG. 1, the device 1-1 is inthe role of the transmitter which transmits information (including userdata) via the communication channel 5 to the communication device 1-2which is in the role of the receiver. Because the devices 1-1 and 1-2are the same, they comprise the same subunits (i.e. the transmittingunits 2-1 and 2-2 are identical, the receiving units 3-1 and 3-2 areidentical and the link controllers 4-1 and 4-2 are identical).Especially, the receiving unit 3-1 comprises the same subunits as areshown for the receiving unit 3-2 and the transmitting unit 2-2 comprisesthe same subunits as are shown for the transmitting unit 2-1. Alloperations the communication device 1-1 is adapted to perform, thecommunication device 1-2 is also adapted to perform and vice versa.Nevertheless, the communication device 1-2 according to the presentinvention may, be adapted to operate in the receiver mode only andcomprise only the subunits required for operating in the receiver mode.When not referring to a special one of the communication devices 1-1 and1-2 or to a subunit of the devices 1-1 and 1-2, the suffix “-2” and “-1”used to differentiate between the receiver and the transmitter may bedropped in the following.

The communication device 1, generally, may be any kind of multi-carrier,multiplexing MIMO communication device. Advantageously, it is anadaptive multi-carrier, multiplexing MIMO communication device, as isthe communication device 1 according to the present embodiment. Inadaptive multi-carrier, multiplexing MIMO communication systems,preferably each subcarrier on each transmission channel is modulatedaccording to the SNR of the subcarrier on the transmission channel.However, not all subcarriers on all transmission channels are requiredto be modulated according to the SNR, at least some subcarriers on sometransmission channels are modulated according to the SNR. For arelatively high SNR, a modulation scheme with a relatively high order isemployed. For a relatively low SNR, a modulation scheme with arelatively low order is employed. For a very low SNR, the subcarrier onthe transmission channel is not used for transmitting data (subcarrieron transmission channel is notched). The communication device 1 may be awired (e.g. a PLC modem) or a wireless (e.g. an RF wireless) MIMOcommunication device. It may be a stationary (e.g. a WLAN base station)or a non-stationary (i.e. portable) (e.g. a mobile phone) communicationdevice.

Now, transmission of data (including user data) from the transmitter 1-1to the receiver 1-2 is explained.

The transmitting unit 2-1 comprises in the order of signal processing aserial-to-parallel converter (S/P) 10, symbol mappers 12, a powerallocator 13, a MIMO precoder 14, and multi-carrier (MC) modulators 16.Therefore, the embodiment of the communication device according to thepresent invention is an adaptive multi-carrier communication device.

The S/P 10 receives a stream of input data. The input data is given inthe form of bits and may comprise user data. The S/P 10 converts(splits) the input data to a number N=>2 of parallel streams, the numberN corresponding to the number of transmit paths T1, T2 and to the numberof parallel and independent transmission channels (decoupled channels)obtained by means of precoding (e.g. Eigenbeamforming). In FIG. 2, N=2holds. The splitting ratio depends on the available capacity (data rate)on each transmission channel. The operation of the S/P 10 is controlledby the link controller 4-1, based on the tonemaps described below.

Each symbol mapper 12 (e.g. QAM modulator 12) corresponds to onetransmission channel (e.g. decoupled transmission channel) and performsa symbol mapping on the received data stream according to constellationinformation (constellation diagrams) provided by the link controller4-1. Each subcarrier may be assigned a different constellation. Theconstellation information for the plurality subcarriers is also called atonemap (e.g. OFDM tonemap). Each symbol mapper 12 uses a separatetonemap. Each symbol mapper 12 puts out one symbol for each subcarrier.The symbols put out by the plurality of symbol mappers 12 for a givensubcarrier form a symbols vector s. The symbol vector s comprises Nsymbols (is of size M. One symbol vectors s is put out for eachsubcarrier. The operation of a symbol mapper 12 is also referred to asmodulation and a constellation specifies (partly at least) a modulationscheme. The plurality of symbol mappers 12 form a symbol mapper unit 12.Thus, for each subcarrier, the transmission channels are modulatedindependently and independent symbols are (at least may be) transmittedon different transmission channels.

The power allocator 13 allocates transmission power to each of thedecoupled transmission channels. The power allocator 13 is in thedigital realm of the transmitting unit 2-1. Power allocation may berealized by multiplying the symbol transmitted on a given subcarrier ofa given transmission channel with an amplification factor (e.g. a realpositive value) which corresponds to the allocated transmit power. Thepower allocator 13 receives transmit power levels based on which theamplification factors can be determined from the link controller 4-1.

The MIMO precoder 14 precodes the symbol vectors s according to amultiplexing MIMO encoding method. The MIMO precoder 14 has input portsfor receiving the symbol vector s and output ports for putting out theprecoded vector. The input ports correspond to the transmission channels(e.g. decoupled transmission channels) and the output ports correspondto the transmit ports T1, T2 (i.e. a symbol that is put out from anoutput port of the MIMO precoder 14 is transmitted—after processing bythe corresponding MC modulator 16—on a corresponding one of the transmitpaths T1, T2). The MIMO precoder 14, for example an Eigenbeamformingprecoder 14, precodes the symbol vector s and puts out a precoded vectorof the same size N for each subcarrier. However, generally, precodingmay be omitted, in this case the transmission channels correspond to theoutput ports T1, T2.

The link controller 4-1 receives the tonemaps (comprising notchinginformation) to use for symbol mapping from the receiver 1-2 via thereceiving unit 3-1. In this embodiment it is assumed, that allinformation that is transmitted from the receiver 1-2 to the transmitter1-1 is transmitted via the transmitting unit 2-2 and the receiving unit3-1. The link controller 4-1 determines the transmit power levels basedon the notching information as is described below. Alternatively or inaddition to receiving notching information, the link controller 4-1 mayreceive the transmit power levels from the receiver 1-2. In this case,it need not calculate the transmit power levels based on the notchinginformation. The notching information is regular part of the tonemaps,since the symbol mappers 12 must know which subcarrier is notched onwhich transmission channel.

Each MC modulator 16 (e.g. OFDM modulator 16) corresponds to one of thetransmit paths (i.e. output ports) T1, T2, receives one symbol of theprecoded vector and modulates (e.g. OFDM modulates) the received symbolso that it can be transmitted on the corresponding transmit path. Incase of OFDM, each MC modulator 16 may, for example, but need not,comprise an IFFT, a DAC and an RF circuit (elements not shown) as isknown in the art. Another example of a MC modulator 16 is a waveletmodulator 16. Other MC modulators 16 than OFDM modulators also may, butneed not, comprise a DAC and an RF circuit.

The multi-carrier (MC) modulated symbols (symbol vectors) aretransmitted via the MIMO transmission channel 5 to the receiver 1-2. Foreach subcarrier, the MIMO channel 5 is described by a separate channelstate information (CSI).

The receiving unit 3-2 comprises in the order of signal processing MCdemodulators 18, a channel estimator 20, a MIMO detector 22, symboldemappers 24 and a parallel-to-serial converter (P/S) 26.

Information corresponding to a transmitted MC modulated (e.g. OFDMmodulated) symbol vector is received by the receiver 1-2 (the receivingunit 3-2) on a number M>=2 of receive paths R1, R2, R3, R4. In FIG. 1,M=4 holds. The received information forms an (MC modulated) receivedsymbol vector of size M. Each MC demodulator 18 (e.g. OFDM demodulator18) corresponds to one of the receive paths R1, R2, R3, R4, receives onesymbol of the (MC modulated) received symbol vector and MC demodulatesthe received symbol according to a MC demodulation method. The MCdemodulation method corresponds to the MC modulation method used in thetransmitter 1-1. The MC demodulated symbols of each subcarrier form areceived symbol vector y. In the case of OFDM, each demodulator 18 may,for example, but need not, comprise an RF circuit, an ADC and a FFT(elements not shown) as is known in the art. Another example of a MCdemodulator 18 is a wavelet demodulator 18. Other MC demodulators 18than OFDM demodulators also may, but need not, comprise an RF circuitand an ADC.

The channel estimator 20 is adapted to calculate channel stateinformation (CSI) for each subcarrier. Channel estimation (i.e.calculation of the CSI) is based on signals transmitted from thetransmitter 1-1 to the receiver 1-2 via the MIMO channel 5. The channelestimation techniques used may, for example, be based on OFDM trainingbursts and pilot symbols.

Channel estimation, pilot symbols and OFDM training bursts are known inthe art and will not be further described here. The channel estimator 20provides the CSI to the link controller 4-2.

The link controller 4-2 determines the tonemaps to be used fortransmission from the transmitter 1-1 to the receiver 1-2, provides thetonemaps to the symbol demappers 24 and transmits the tonemaps to thetransmitter 1-1 via the transmitting unit 2-2. State of the art methodsfor obtaining the tonemaps may be used. For example, the link controller4-2 may determine the modulation scheme that is to be used by a givensubcarrier on a given transmission channel (e.g. decoupled transmissionchannel) based on the indicator value of the SNR of the given subcarrieron the given transmission channel. For a high SNR, a high ordermodulation scheme is chosen and for a low SNR, a low order modulationscheme is chosen. If the indicator value of the SNR is below a thresholdvalue, the given subcarrier on the given transmission channel is notused for transmitting data. The given subcarrier on the giventransmission channel is said to be notched. The information aboutnotched subcarriers is part of the tonemaps. The link controller 4-2 maydetermine a multiplexing MIMO precoding method (e.g. an Eigenbeamformingprecoding matrix) and transmit corresponding information (e.g. an indexof an entry of a codebook comprising precoding matrices as entries)describing the precoding method to the transmitter 1-1 via thetransmitting unit 2-2.

The MIMO detector 22 performs a detection on the received symbol vectory and obtains an estimates of the symbol vector s for each subcarrierbased on the corresponding channel state information and the selectedprecoding method (e.g. precoding matrix). The MIMO detector 22 has inputports for receiving the received symbol vector y and output ports forputting out the detected estimate s . The input ports correspond to thereceive paths R1, R2, R3, R4 (i.e. a symbol that is received on one ofthe receive paths R1, R2, R3, R4 by the receiver 1-2 is input—afterprocessing by the corresponding MC demodulator 16—to the MIMO detector22 on a corresponding one of the input ports of the MIMO detector 22)and the output ports correspond to the (decoupled) transmissionchannels. Detection is also known as decoding, and the MIMO detector 22may also be called a MIMO decoder 22. When expressed in terms ofdecoding, the MIMO decoder 22 decodes the received symbol vector ythereby obtaining the estimates of the symbol vector s which iscomprised in the received symbol vector y in encoded form (s is encodedin y). The MIMO detector 22 may, for example, be a Zero Forcing (ZF)detector, a Minimum Mean Square Error (MMSE) detector and a MaximumLikelihood (ML) detector, but other detectors are possible too.

Each symbol demapper 24 (e.g. QAM demodulator 24) corresponds to onetransmission channel and demaps (e.g. QAM demodulates) to the receivedsymbols according to the tonemaps (constellation information) providedby the link controller 4-2. The symbol demapping operation correspondsto the symbol mapping operation in the corresponding symbol mapper 12 inthe transmitter 1-1. The operation of a symbol demapper 24 is also knownas demodulation. The plurality of symbol demappers 24 form a symboldemapping unit 24. After the symbol demapping unit 24, the informationcorresponding to the symbol vectors for each subcarrier is provided inthe form of bits.

The P/S 26 serializes the output bits of the symbol demappers 24 andprovides them as a single stream of output data. When data transmissionis successful, the output data is identical to the input data. However,it is not required that all bits are transmitted successfully, sinceerror correction methods may be applied in the transmitter 1-1 and thereceiver 1-2.

FIG. 2 shows a flow diagram of a first embodiment of the method ofassigning transmit power to two or more communication channels. Themethod is carried out by the communication system 6. The twotransmission channels have preassigned the same transmit power. When atransmission channel is not used for transmitting data, the preassignedtransmission power is completely moved from the transmission channel tothe other transmission channel. This corresponds to doubling thetransmission power (i.e. a 3 dB increase of transmission power).

The increase of transmission power leads to an increased SNR at thereceiver 1-2. When the increase of transmission power and the SNR areexpressed in dB (decibel), then the increase of the SNR is the same asthe increase of the transmission power. Here, the SNR of the non-notchedtransmission channel is increased by 3 dB at the receiver 1-2.

In a step S2, a first subcarrier is selected. The method proceeds to astep S4.

In step S4, it is determined if the first transmission channel isnotched at the selected subcarrier (i.e. it is determined if theselected subcarrier is not used for transmitting data on the firsttransmission channel). If yes, the method proceeds to a step S10. If no,the method proceeds to a step S6.

In step S6, it is determined if the second transmission channel isnotched at the selected subcarrier (i.e. it is determined if theselected subcarrier is not used for transmitting data on the secondtransmission channel). If yes, the method proceeds to a step S8. If no,the method proceeds to a step S 16.

In step S8, the second transmission channel is assigned zero transmitpower (the preassigned transmit power is removed completely) and thefirst transmission channel is assigned 3 dB more transmit power.Thereafter, the method proceeds to a step S 16.

In step S10, it is determined if the second transmission channel isnotched at the selected subcarrier (i.e. it is determined if theselected subcarrier is not used for transmitting data on the secondtransmission channel). If yes, the method proceeds to a step S14. If no,the method proceeds to a step S 12.

In step S12, the first transmission channel is assigned zero transmitpower (the preassigned transmit power is removed completely) and thesecond transmission channel is assigned 3 dB more transmit power.Thereafter, the method proceeds to a step S 16.

In step S14, the preassigned transmission power is removed completelyfrom both transmission channels (the first transmission channel isassigned zero transmit power and the second transmission channel isassigned zero transmit power). Thereafter, the method proceeds to a stepS16.

In step S16, it is determined if all subcarriers have already beenconsidered (assigned/deassigned). If no, the method proceeds to a stepS18, selects the next subcarrier, and returns to step S4. If no,assignment is complete and the method ends.

It must be noted, that the steps S10 and S14 are optional. When omitted,the method may directly proceed to step S12, if in step S4 the channelis determined to be notched. They are provided for explanation purposeand may be used for optional transmit power bookkeeping purposes.

FIG. 3 shows an example of the increase of SNR according to the abovemethod. FIG. 3 a) shows the SNR of a first transmission channel obtainedby Eigenbeamforming. FIG. 3 b) shows the SNR of second transmissionchannel obtained by Eigenbeamforming. It is typical that the onetransmission channel obtained by Eigenbeamforming is much stronger thanthe other transmission channels. It can be seen that in regions wherethe second transmission channel is notched, the SNR of the firsttransmission channel is increased by 3 dB when compared to classicalEigenbeamforming. A combination of Eigenbeamforming with waterfilling(no SNR curve is shown for this combination) yields approximately thesame SNR as pure Eigenbeamforming. Because the SNR is increased, thechannel capacity is increased. Therefore, the data rate can beincreased. The data rate can be increased, for example, by selecting amodulation scheme of a higher modulation order than would be possiblewithout the increased SNR and/or by selecting an error correction schemewith less encoding overhead (i.e. with a smaller ratio of encoded tounencoded bits).

In case more than two transmission channels are present, the power ofthe notched transmission channel(s) may, for example, be assignedequally to the non-notched transmission channels. Generally (i.e. incase of two or more transmission channels), it is advantageous when allpower of a notched transmission channel is assigned to the non-notchedtransmission channels, since this provides for the maximum increase ofthe SNR. After all notched channels have been considered, thenon-notched transmission channels have assigned an overall transmitpower which is given by the sum of the preassigned transmit power andthe additional transmit power assigned to the channel. Of course, alldeclarations of this paragraph are relative to a given (selected)subcarrier. For example, advantageously, all power of a notchedsubcarrier on a transmission channel is assigned to the correspondingnon-notched subcarriers (of same frequency) on the other transmissionchannels. When a non-notched transmission channel is assigned transmitpower, it is assigned to the same subcarrier of the transmission channelas the subcarrier of the transmission channel from which the transmitpower is taken away.

Because the power allocator 13 applies an amplification corresponding tothe assigned transmit power, it is obvious that the transmitter 1-1 mustknow about the determined transmit power levels. However, there existcommunication schemes, where the receiver 1-2 must also know about theassigned transmit power levels. For example, when the channel estimationis performed based on training symbols that are transmitted with apredefined transmit power (e.g. the preassigned transmit power), (whichpossibly is different from transmit power levels applied later for datasymbols), the receiver 1-2 must know about the transmit power levels inorder to correctly decode data symbols (at least when an amplitude basedmodulation method is employed). Also, in case the receiver 1-2determines tonemaps, it is advantageous when the receiver 1-2 knowsabout the transmit power levels.

Thus, in an embodiment, the above method (steps S2 to S18) is executedin the receiver 1-2 and in the transmitter 1-1.

When executed in the receiver 1-2, the method may be executed by thelink controller 4-2. FIG. 4 shows a method corresponding to a possiblerealization of the method of FIG. 2, when executed in the receiver 1-2.The method of FIG. 4 is the same as the method of FIG. 2, except thatsteps S4, S6 and S10 are replaced (realized) by steps S4-2, S6-2 and S10-2, respectively. Description of the other steps is therefore omitted.

In the step S4-2, it is determined if the SNR of the selected subcarrieron the first transmission channel is below a threshold. If yes, thefirst transmission channel is determined to be notched (at the selectedsubcarrier) and the method proceeds to step S 10-2. If no, the firstsubcarrier is determined not to be notched (at the selected subcarrier)and the method proceeds to step S6-2.

In the step S6-2, it is determined if the SNR of the selected subcarrieron the second transmission channel is below a threshold. If yes, thesecond transmission channel is determined to be notched (at the selectedsubcarrier) and the method proceeds to step S8. If no, the secondtransmission channel is determined not to be notched (at the selectedsubcarrier) and the method proceeds to step S16.

In the step S10-2, it is determined if the SNR of the selectedsubcarrier on the second transmission channel is below a threshold. Ifyes, the second transmission channel is determined to be notched (at theselected subcarrier) and the method proceeds to step S14. If no, thesecond transmission channel is determined not to be notched (at theselected subcarrier) and the method proceeds to step S12.

It is noted that it is equivalent to say “a subcarrier is notched on atransmission channel”, “a subcarrier on a transmission channel isnotched” and “a transmission channel is notched at a subcarrier”.

For the above determination if the SNR is below a threshold, anindicator value of the SNR may be used. It depends on the definition ofthe indicator value of the SNR, if an indicator value of the SNR that isabove a threshold or an indicator value of the SNR that is below athreshold correspond to a SNR that is below a threshold.

Determination of notched subcarriers is part of the determination of thetonemap for each transmission channel executed by the link controller4-2. The tonemaps comprise the notching information indicating whichsubcarrier is notched on which transmission channel. The link controller4-2 transmits the tonemaps via the transmitting unit 2-2 to thetransmitter 1-1. The link controller 4-1 receives the tonemaps via thereceiving unit 3-1 and calculates the transmit power levels according tothe above method (steps S2 to S18) of FIG. 2.

FIG. 5 shows a method corresponding to a possible realization of themethod of FIG. 2, when executed in the transmitter 1-1. The method ofFIG. 5 is the same as the method of FIG. 2, except that steps S4, S6 andS10 are replaced (realized) by steps S4-1, S6-1 and S10-1, respectively.Description of the other steps is therefore omitted.

In step S4-1, it is determined if the tonemap (notching information) ofthe first transmission channel indicates the selected subcarrier to benotched. If yes, the first transmission channel is determined to benotched (at the selected subcarrier) and the method proceeds to stepS10-1. If no, the first transmission channel is determined not to benotched (at the selected subcarrier) and the method proceeds to stepS6-1.

In step S6-1, it is determined if the tonemap (notching information) ofthe second transmission channel indicates the selected subcarrier to benotched. If yes, the second transmission channel is determined to benotched (at the selected subcarrier) and the method proceeds to step S8.If no, the second transmission channel is determined not to be notched(at the selected subcarrier) and the method proceeds to step S16.

In step S10-1, it is determined if the tonemap (notching information) ofthe second transmission channel indicates the selected subcarrier to benotched. If yes, the second transmission channel is determined to benotched (at the selected subcarrier) and the method proceeds to stepS14. If no, the second transmission channel is determined not to benotched (at the selected subcarrier) and the method proceeds to stepS12.

Thus, when the method is executed in the transmitter 1-1, it isdetermined in each of the steps S4, S6 and S10 if the tonemap (notchinginformation) of the given transmission channel indicates the givensubcarrier to be notched. If yes, the transmission channel is determinedto be notched (at the given subcarrier). If no, the transmission channelis determined not to be notched (at the given subcarrier).

A flow diagram incorporating the above teaching is given in FIG. 6. Theflow diagram corresponds to a further embodiment of the method ofassigning transmit power to two or more communication channels accordingto the present invention.

In a step S30, an indicator value of the SNR is determined for eachsubcarrier of each transmission channel by the receiver 1-2 (e.g. thelink controller 4-2). Determination of the indicator value of the SNR isbased on one or more signals received by the receiver 1-2 from thetransmitter 1-1. The link controller may hereby make use of the CSIprovided by the channel estimator 20.

In steps S32 and S34, the link controller 4-2 determines the notchedsubcarriers on each transmission channel based on the determinedindicator values of the SNR and determines the transmit power level (atleast the additional transmit power) for each subcarrier on eachtransmission channel. Steps S32 and S34 may be realized by executing themethod of FIG. 4 (receiver case).

In a step S36, the link controller 4-2 determines an indicator value ofan expected SNR for each (non-notched) subcarrier on each transmissionchannel based on the corresponding indicator value of the SNR determinedin step S30 and the corresponding transmit power level (additionaltransmit power). When the values are expressed in dB (dezibel), theexpected SNR is the sum of the SNR and the additionally assignedtransmit power. In the example given above, the expected SNR is 3 dBhigher than the SNR (in case additional power has been assigned).

In a step S38, the link controller 4-2 determines a tonemap for eachtransmission channel based on the respective indicator values of theexpected SNR. Notched subcarriers are indicated in the tonemaps.

In a step S40, the link controller 4-2 configures the receiving unit 3-2(e.g. the MIMO detector 22 and/or the symbol demappers 24) so as toprocess signals received from the transmitter 1-1 based on thedetermined transmit power levels.

In a step S42, the link controller 4-2 configures the symbol demappers24 with the respective tonemaps, so as to apply the correct demodulationfor received signals.

In a step S44, the link controller 4-2 transmits the tonemaps to thetransmitter 1-1 via the transmitting unit 2-2. The link controller 4-1receives the tonemaps via the receiving unit 3-1.

In a step S46, the link controller 4-1 determines the transmit powerlevels for each (non-notched) subcarrier on each transmission channelbased on the received tonemaps. This step may be realized by executingthe method of FIG. 5 (transmitter case).

In a step S48, the link controller 4-1 configures the power allocator 13so as to apply the determined transmit power levels.

In a step S50, the link controller 4-1 configures the symbol mappers 12with the respective tonemaps, so as to apply the determined modulationscheme for each subcarrier and transmission channel.

In an alternative embodiment, the link controller 4-2 transmits thecalculated transmit power levels via the transmitting unit 2-2 to thetransmitter 1-1. The power allocator 13 then applies the receivedtransmit power levels.

1. A method of assigning transmit power in a MIMO two-streamtransmission, comprising receiving first and second symbol streams, thefirst symbol stream associated with a plurality of subcarriers inaccordance with constellation information contained in a first tonemapand the second symbol stream associated with the plurality ofsubcarriers in accordance with constellation information in a secondtone map, if, according to the first tonemap, a subcarrier is not usedfor transmitting data for the first symbol stream and if, according tothe second tonemap, that subcarrier is used for transmitting data forthe second symbol stream, allocating, for that subcarrier, a preassignedtransmit power portion for the first symbol stream to the second symbolstream; and if, according to the second tonemap, a subcarrier is notused for transmitting data for the second symbol stream and if,according to the first tonemap, that subcarrier is used for transmittingdata for the first symbol stream, allocating, for that subcarrier, apreassigned transmit power portion for the second symbol stream to thefirst symbol stream.
 2. The method according to claim 1, wherein afterallocating the transmit power portion to the first or second symbolstream, respectively, the transmit power portion for the first or secondsymbol stream is 3 dB more than the preassigned transmit power portionfor the first or second symbol stream, respectively.
 3. The methodaccording to claim 1, wherein for each of the plurality of subcarriers,if according to the first and second tonernaps one of the first andsecond symbol streams is not used for transmitting information at thesubcarrier and the other one of the first and second symbol streams isused for transmitting information at the subcarrier, a preassignedtransmit power portion for the one of the first and second symbolstreams that is not used for transmitting information at the subcarrieris assigned to the other one of the first and second symbol streamswhich is used for transmitting information at the subcarrier.
 4. Themethod according to claim 1, wherein allocating a transmit power portionincludes multiplying a symbol of the first or second symbol stream,respectively, with an amplification factor corresponding to theallocated transmit power portion.
 5. The method according to claim 1,wherein the MIMO two-stream transmission is a PLC MIMO two-streamtransmission.
 6. A MIMO communication device, comprising a first symbolmapper adapted to receive a first data stream, performing a symbolmapping on the first data stream according to a first tone map for aplurality of subcarriers, and outputting a first symbol stream, a secondsymbol mapper adapted to receive a second data stream, performing asymbol mapping on the second data stream according to a second tone mapfor the plurality of subcarriers and outputting a second symbol stream,and a power allocation unit adapted to receive the first and secondsymbol streams and to assign, for a subcarrier, if according to thefirst and second tonemaps one of the first and second symbol streams isnot used for transmitting information at the subcarrier and the otherone of the first and second symbol streams is used for transmittinginformation at the subcarrier, a preassigned transmit power portion forthe one of the first and second symbol streams that is not used fortransmitting information at the subcarrier to the other one of the firstand second symbol streams which is used for transmitting information atthe subcarrier.
 7. The MIMO communication device according to claim 6,wherein by assigning the preassigned transmit power portion for thesymbol stream that is not used for transmitting information at thesubcarrier to the one of the first and second symbol streams which isused for transmitting information at the subcarrier, the preassignedtransmit power portion of that symbol stream is increased by 3 dB. 8.The MIMO communication device according to claim 6, wherein the powerallocation unit is further adapted to assign, for each of the pluralityof subcarriers, if according to the first and second tonemaps one of thefirst and second symbol streams is not used for transmitting informationat the subcarrier and the other one of the first and second symbolstreams is used for transmitting information at the subcarrier, apreassigned transmit power portion for the one of the first and secondsymbol streams that is not used for transmitting information at thesubcarrier to the other one of the first and second symbol streams whichis used for transmitting information at the subcarrier.
 9. The MIMOcommunication device according to claim 6, wherein the power allocationunit is further configured to multiply a symbol of the first or secondsymbol stream, respectively, with an amplification factor correspondingto the allocated transmit power portion.
 10. The MIMO communicationdevice according to claim 6, wherein the MIMO communication device is aPLC MIMO communication device.